There is a need to be able to acquire image information representing the interior spaces of various forms of structures, including the interior structures, layout and contents of the structure and the spaces therein and wherein the structures may comprised of a variety of materials, including, for example, concrete, brick, steel, wood, rock and other earth materials, such as mud brick, artificial materials such as plastics, and so on.
There are, however, a number of problems with attempts to obtain such images due, for example, to the relative impermeability or opacity of the structural materials, interference from various sources and difficulties in obtaining image data having usable resolution or discrimination. For example, acoustic and visible or infra-red energy has often be used in listening systems to detect the effects of internal sounds on externally accessible portions of a structure, such as walls or windows. Such systems are not useful for imaging of internal spaces, however, because acoustic and visible or infra-red energy is readily blocked or absorbed by most building materials and is subject to interference from other energy sources, such as any source of extraneous noise or heat radiated from parts of the structure itself.
For these reasons, most more recent attempts at imaging the internal spaces of structures have employed ultra-wide band (UWB) radar systems, that is, systems that generate images from backscatter from an ultra-wide band signal in the radar frequency spectrums. Such systems are advantageous in that radio and radar frequency signals penetrate most structural materials more readily than do higher and lower frequency signals, thus allowing a stronger possible returned signal. In addition, the variation in the absorption, reflection and refraction characteristics of various materials is greater in the radar frequency spectrum than in higher or lower frequency spectrums, thereby providing greater possible discrimination between various materials.
It must be noted, however, that most typical building materials are still highly absorptive of radar and high frequency radio signals, and that the absorption of the signals increases with the signal frequency. The image resolution of such systems, however, is dependent upon the signal frequency components with higher resolutions requiring higher frequency signal components. For example, conventional radar frequency backscatter imaging systems transmit a series of radar frequency pulses wherein the image resolution is determined by the pulse width. Many current radar backscatter imaging systems, for example, provide an image resolution on the order of one meter, which results in a pulse having a lowest frequency component wavelength of one meter and significant higher frequency components that are multiples of the lowest frequency component.
In this regard, it must be noted that the number and amplitude of the higher frequency components increases rapidly as the pulse width decreases, so that current systems having at least currently acceptable image resolutions are ultra-wide band systems, that is, systems transmitting and receiving signals occupying very wide frequency bands. Ultra-wide band systems are disadvantageous in this regard, however, in that the blocking and absorption of the signal components by most conventional building materials increases rapidly as the frequency of the components increases. As a consequence, the effective range of such systems are typically very limited given the signal power achievable in reasonably portable systems.
In addition, and because ultra-wide band systems must receive signals over ultra-wide frequency bands, there is a significantly increased possibility that the system bandwidth will overlap one or more extraneous signals, such as signals from television stations and other radars, that may be at least as strong and often stronger than the backscatter return signal and that can seriously disrupt or degrade the received image data.
The present invention addresses these and other problems of the prior art.